Dimpling



United States Patent O DIlVIPLING Everett Chapman, Marshallton, Pa., assiguor of one-half to Aeroprojects Incorporated, West Chester, Pa., a corporation of Pennsylvania v Application February 5, 1951, Serial No. 209,487

Claims. (Cl. 78-81) The present invention relates to apparatus and method for dimpling sheet metal and more particularly to apparatus and method for dimpling sheet metal characterized by high strength and low elongation.

Heretofore existing apparatus and methods for forming crack-free dimples in high strength-low ductility sheet metal have not adequately provided for the criticality of sheer strength and desired configuration in dimples formed in the aforesaid metal.

Efforts have been made to provide tools and methods by which high-strength dimples can be formed, but such efforts have not been successful due to the` fact that the deformation of the metal in the forming of the dimples induced a weakness resulting from cracking, thinning, and the creation of hightension areas in the deformed portions of the metal. Furthermore, the deformation of the` metal in the forming of the dimples has not heretofore been sufficiently controlled to provide a desred dimple configuration especially with regard to the peripheral crest of said dimples wherein it is highly desirable that the contour of the dimple crest be sharply defined and in the plane of the metal sheet.

In forming the dimple, e. g. cold forming, a bore is preferably first formed through a metal sheet and subsequently an area around said bore is dimple punched. In the punching operation, the area around said bore is bent within safe limits, e. g. limits which do not create an excessive tension in the bent area which is potentially inducive to the formation of cracks therein. Having dimple punched the sheet metal in such manner which constitutes a first stage, neither the crest contour nor the angulation of the dimple is desirable and a further deformation or second stage is necessary. It is with such further deformation that undesirable stresses and strains manifest themselves and it is particularly with such further deformation that various means and methods have been proposed in the formation of dimples.

Among the several proposals for dimpling low ductility sheet metal are means and methods' for coining the deformed areas and means and methods for heat treating said deformed areas. The former proposed means and methods are directed to relieving the tension in the bent area by producing a counteracting compression, but which compression is not adequate in that the tension created in the vicinity of the peripheral crest of the dimple is merely decreased but not sufficiently counteracted to prevent at least some cracking, e. g. internal sheer cracks in the zone of deformation away from the plane of the sheet metal. The cracking potential of inadequate compression increases as the ductility of the work material decreases and proper compression becomes particularly important in the brittle materials at high strength and low elongation, e. g. in materials such as hardened aluminum or magnesium alloys. Among such alloys are:

Mg Remainder ICC The latter proposed means and methods involve the use of heated tools which are particularly applicable to especially hard metals. However, heating these metals introduces a compromise between eas'e in formation of the dimple and the strength of the dimple. The heating of such metals necessitates a critical annealing temperature which must be sufficient to prevent the formation of cracks and still not great enough to disadvantageously weaken the metal. For example, heating these metals too hot will result in a loss of strength, since the metals are previously heat treated or hard-rolled for maximum strengths and excessive temperatures will anneal and soften the metal so that the best physical properties are no longer present, and some of the alloying elements may be precipitated to create regions which would be subject to intergranular corrosion.

The above mentioned compromise may be advantageous in certain instances with, however, sacrices in portability of the equipment and the ability to use small tools in restricted places. The hazard of working with hot tools, the use of expensive and complicated control equipment and the time element, are objectionable where rapid and economical dimpling is preferred.

It is an object of the present invention to provide a method and means therefore which provide a superior dimple of great strength and desirable configuration It is another object of the present invention to provide a method and means for the economical and rapid formation of dimples. It is another object of the present invention to provide a superior dimple by the use of simple solid tools. It is a further object of the present invention to provide a method and means for forming dimples in hard metals without the necessity of using heated tools. It is a still further object of the present invention to provide a method and means for forming dimples and whereby the deformation of a sheet metal in the forming of said dimples is controlled to substantially inhibit the creation of high tension areas in said metal. Other objects and advantages of the present invention will become apparent from the description hereinafter following and the drawings forming part hereof, in which:

Figs. 1 through 3 are schematic illustrations of a punch and die in a first stage operation according to the present invention, and

Figs. 4 and 5 are schematic illustrations of a punch and die in a second operation according to the present mventlon.

In bending metal, especially metal of high strength and low ductility, around a corner to a sharp radius, cracking and the attendant loss of strength can be prevented if a compressive neutralizing force can be introduced against a tension region or area created by the operation.

In the forming of a dimple in a sheet of metal, crackless deformation can be achieved if the formation of a tension area is inhibited by the use of a punch and die shaped so that the metal undergoing deformation isl directed back against the region which would otherwise be in tension. Under such conditions, large deformations can be produced cold, without cracks, in materials that are low in ductility.

In directing the metal undergoing a dimpling deformation to counteract the tension producing potential according to the present invention, the compressive forces introduced by the punch and die are multiaxial and promote a plastic flow of metal in the direction of the dimple crest to provide a region, including said dimple crest, which is in a state of high residual compression. The residually compressed region is very amenable to further crackless deformation since the punch and die of the invention are so shaped that a substantially large amount of plastic flow, which created said residually compressed region, is available for further deformation under compression without the attendant disadvantages of deformation characteristic of metal under tension.

Figs. 1 through 3 schematically illustrate a rst stage operation in the deformation of a metal sheet in the formation of a dimple according to the present invention. In order to clarify the invention, Fig. l is introduced as a somewhat exaggerated illustration directed primarily toward the theoretical consideration of multiaxial forces involved Yin the first dimpling stage. `For example, when punch ll is forced into a sheet of metal 2 which is supported by a fiat anvil 3, a volumetric displacement occurs, due to a plastic ilow of metal under compression, such that a Volume of compressed sheet metal A is displaced outwardly of punch lt and becomes localized at B outwardly or' and around the depression formed by the punch l. This displacement occurs entirely under compression and the compressed displaced metal B is characteristically tension-free and as such cannot tear. The forces acting on and within the sheet 2 and which result in this particular displacement of metal A to its new locale B are the normal opposing forces Ei and E2 which produce a resultant force F in the direction of least resistance and in which direction the plastic how of compressed metal is directed. lt is apparent from the illustrations that part of the displaced metal B is in the form of a raised annulus around the depression formed by the punch and which is created by the outward radial plastic iiow of metal under the influence of the resistance R disposed radially inward and a'orded by the stiine'ss of the sheet 2 surrounding the 'region of flow. Therefore, a raised annulus is formed outwardly of the depression by the displaced metal.

The punch and die of the present invention are so shaped that the plastic flow of metal as above described is not choked by any throat constriction, but, on the contrary, the radial extrusion throat area is increased in a direction away from the punch and the metal flows into a more and more open channel, e. g. a channel being radially symmetrical about a common punch-die axis and having right cross sections characterized as annuli with the radial width of said annuli increasing in a direction away from the end of said punch. Furthermore, the plastically displaced metal including said annulus is in a state of locked-up residual compression stresses and, therefore, capable of far more deformation plastically than if the residual stress system was zero or under tension.

Fig. 2 departs from the structure of Fig. l only in that the theory above outlined is applicable and practical when the fiat anvil 3 -is replaced with a die 4. However, the relationship between the punch and die is of utmost importance. The relationship is critical in that the dimpling punch l -a'nd the dimpling die 4 are unmated'during the rs't stage operation, i. e. the punch l comprises an end portion having walls unmated with the die of female configuration, e. g. a die compris-ing a coned countersink or an arcuate depression, so that from a common axis 5 the walls 6 of the die are of greater divergence than the walls 7 of the punch which may be radially convex or otherwise rounded, said difference of divergence from said axis providing the radial extrusion throat above set forth and directing the plastic flow of metal as heretofore described.

Fig. 3 illustrates a modification in the first s't'age dimple forming loperation whereby the 'punch S is provided with tapered sides V9, said sides being applicable and cooperative with a die 10 wherein the arcuate or coned depression is of substantial depth or of depth in conformity with the n desired angularity of the completed dimple. The tapered sides `9 of the punch 8, similarly to the walls of the punch described with regard to Fig. 2, are of lesser divergence from a common punch-'die axis than the walls of the die and maintain the aforesaid relationship between punch and die to direct a plastic iiow of metal under compression having a tendency to form a raised annulus yor with the consequent formation of an an'nulus as hereinbefore set forth, whereby the displaced metal including said annulus is in a state of locked-up residual compression stresses and capable `of further plastic deformation substantially free of vtension stresses. -In this modification, since the depression or countersink of the die is of substantial depth, a new force lS will Iappear in a direction opposing the desired compressive force F. The new force S represents a movement of metal necessary Lto contact the walls of the depression, said movement being normally a tension producing movement, but, due to the fact that the tapered sides 9 act primarily upon the displaced metal, i. e. the annular compressed metal, lin order to contact the walls of the depression with sheet metal, tension is inhibited since only compressed metal is 'plastically moved to-fill the depression. -A prime consideration in the contacting of the walls of "the die ldepression 'with sheet metal is `that the punch yand die are so shaped to actually provide an excess of compressed nietal 1in the form of -said aniiulus as above described so that the forcing of the metal into the die by the tapered sides 9 does not consume all compressed metal and the potentially tension producing force S is, therefore, less than the compressive force F with locked-up residual compression being still present in the deformed metal.

Figs. 4 and 5 illustrate 'the critical second dimple forming stage of the invention. rHaving completed the rst stage operation with the punch and die described, at least a second punch is necessary, or a second punch and a second die, to complete the final configuration of the metal in the forni of a dimple. The second punch il, according to Eig. 4, is cooperative with a vdie Ml as shown in Fig. 3. The punch lll is essentially suchpthat an end thereof is provided with dimple (forming walls of greater divergence trom a common punch ldie `axis than the dimple forming walls of the punch employed in the first stage operation, e. g. the walls of the second punch, as a preferred embodiment, mate with the walls of the second voperation die. Since the second punch mates the die, it is apparent that the punch dimple-forming walls act upon the compressively displaced metal of the first stage operation to plastically form the final coniiuration of the dimple. Where a second punch ll and a second die 13 are employed for the second dimple-forming operation, Ythe requirements of the second punch are as above described and the second die is employed in such instances where the first stage die was 'not of suflicient depth to provide the final ydimple configuration. For example, a second die is necessary if the first stage operation employed a die according to Fig. 2 whereas it would `be unnecessary if l-a 'die according to Fig. 3 was utilized. -lt is, therefore, obvious that two punches are essential 'and that ythe use of a 'second 'die is matter 'of choice depending upon the metal being dimple-.punched or such other requirements wherein one form of die may be preferred over the other.

The punches and dies herein set forth `are schematically illustrated and it is within :the scope of the invention lto provide the sheet metal worked upon with bores formed therethrough so tha-t the area around the bores -are Edimple punched as is the common practice. It is valso Within the scope of Athe invention to .provide said .punches `and dies to cooperate with afpilot or guide to maintain alignment where alarnination of bored metal sheets are dimplepunched. However, the Apilo't'or guide means is absolutely unessential in the 'deformation of lt'he `metal and `does not contribute in the deformation to attain a multiaxial conipression -as described.

Since the punches and ydies -above set forth act on imeta'l in compression, there is no bendingof the sheet metal at the periphery or dimplecrest'in the second Ioperation `and the result is that the dimple 'crest itself -is formed under compression :to provide a 'sharp and clearly'deinedfperiphery `in the plane lof vthe dimpled :sheet nietal.

The advantages of a sharp and `clearly defined dimple periphery is well-known 'in the art in that lsaid dimple may contain the head of, -for example, a rivet, kwith the rivet head exactly tittingvsaid idimple and leaving no departure from the plane of lthe sheet metal to provide a spacing bet-Ween "the rivet 'head and the sheet metal.

AIt is, therefore, 'apparent that `the present invention Sin general deals 'with a `method and apparat-us ltherefore whereby a rst hit dimpling operation utilizes 'simple solid tools which compressively displace a metail Tinto a selected position from which arsu-bsequent1seco'nd hit operation can continue to act `on the metal f-left 'in compression to provide a 'final dimple configuration f-'ree'o-f-oracks'and therefore of high vsheer Strength together `-with a desired dimple crest :heretofore unattainable.

What I 'claim is:

l. The method lvof dimpl-ing fshe'et metal comprising forming a bore through fsa-id sheet metal, ycomp're'ss'ively displacing metal around said bore upwardly andoutwa'rdly thereof and thereafter compressively forming the compressed metal in the form of a frusto-co'nical dimple while lsiri'iillta'rieously compressing Said outwardly displaced Hmetal into-the plane of said shet mtal.

-2. The method of dimpling sheet metal comprising forming a bore therethrough, inderiting the area around said 'bore against the countersurik walls of a die and in the vforir'i of a dimple and simultaneously displacing Athe walls of Ysaid vdimple vupwardlyand outwardly thereof, and then compressing said `outwardly displaced-metal Sinto the plane of said sheet-metal.

3. The fmethod of dimpling sheet `metal comprising forming a bore therethrough, indenting an area of metal around said bore against the countersunk walls of a die thereby forming a dimple and simultaneously displacing the walls of said dimple upwardly and outwardly thereof forming a crest around said dimple, and then compressing said crest into the plane of said sheet metal and said dimple walls into frusto-conical dimple form.

4. The method of dimpling sheet metal according to claim 3, comprising compressing said crest into the plane of said sheet metal and simultaneously compressing said dimple walls into a frusto-conical dimple form.

5. The method of dimpling laminated sheet metal comprising forming a bore therethrough, indenting an area.

of metal around said bore against countersunk walls of a die thereby forming a dimple and simultaneously displacing the walls of said dinjlple upwardly and outwardly thereof forming a crest around said dimple, and subsequently compressing said displaced metal including said crest into a finished frusto-conical form, whereby said crest is compressed into the plane of said sheet metal.

References Cited in the le of this patent UNITED STATES PATENTS Number 

